Existing comb drive biaxial scanners typically operate very close to resonance due to the weaker drive torques and due to the limited scan region where the combs have torque input. This still allows for using these biaxial, biresonant scanners in applications such as imaging and displays. However, it is generally challenging to make the slow scan resonant frequency low enough to permit a scanning pattern that produces all the fast scan lines within one slow scan frame. Instead, a lissajous pattern may be created where several slow scan cycles are utilized to create a single complete frame. However, such an arrangement leads to a motion artifact caused by the user's eye motion enabling the brain to detect an impartial frame pattern. One solution to this problem is to reduce the slow scan resonant frequency. In existing comb drive scanners, both axes may be suspended by torsional flexures. In order to reduce the slower scan resonance, the designer must lengthen the flexures, make the flexures thinner, or increase the mass of the moving frame. Such options may cause the scanner to be susceptible to environmental accelerations. The displacement x under a static acceleration load g is inversely proportional to the square of the resonant frequency ω:x=g/ω2 Because of the displacement, these accelerations can cause the combs to crash into one another. Such crashing may cause the fingers to stick, thereby preventing motion and/or creating an electrical short that could damage the device when the voltage is applied across the interdigitated comb fingers. Furthermore, the thinner flexure and/or longer flexure arrangements also can cause the combs to crash due to the resulting softness of the scanner motion in the lateral direction. Thus, it may be difficult to make the slow scan torsional flexures stiff enough to resist the lateral motions that lead to comb crashes while simultaneously making them soft enough to achieve a desired lower frequency.
For higher resolution display applications, the high horizontal axis scan angle and scanning frequency may lead to high energy losses through aerodynamic damping. The damping forces and torques may be generated by airflow around the comb fingers and by air drag on the mirror itself. For a resonant system, the amount of energy input to the system should balance the damping energy loss, so stable steady state scanning mirror oscillations may exist when the energy input per cycle equals the energy loss per cycle. The rapid damping increase with scan angle leads to high input energy requirements for comb drive scanning mirror based display systems. As an example, the drive amplitudes estimate for a 10° mirror scan angle (MSA) scanning at 32 kHz for Super Video Graphics Array (SVGA) display resolution is 276 V. Such higher voltage amplitudes may be an impediment to system integration and miniaturization.
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